Recently, a size of semiconductor circuit pattern becomes smaller as an integration degree of a large scale integrated circuit (LSI) increases. In the conventional lithography technique using a light exposure, a resolution of a resist or circuit pattern is nearly reduced to an intrinsic resolution which results from a wavelength of the used light. As the conventional light source for the lithography technique for forming the resist pattern, a g-line (436 nm) light source using a mercury lamp and an i-line (365 nm) light source are widely used. Recently, a short wavelength light source such as a KrF excimer laser (248 nm) and a ArF excimer laser (193 nm) is used for the lithography technique for reducing the size of the resist pattern.
Also, as the size of the resist pattern is reduced, the thicknesses of a photoresist layer and the resist pattern should be reduced to prevent collapse of the photoresist pattern. However, it is difficult to properly etch a layer (a substrate to be etched) by using the photoresist pattern having a reduced thickness. Therefore, an inorganic layer or an organic layer having a high etching resistance is formed between the photoresist layer and the substrate to be etched, and such layer is called “(resist) under-layer” or “hard mask”. In the substrate etching process, the under-layer is firstly etched and patterned by using the photoresist pattern as a mask, and then the substrate is secondly etched and patterned by using the under-layer pattern as a mask. Such etching process is conventionally called “(resist) under-layer process”. The inorganic under-layer used in the under-layer process can be composed of silicon nitride, silicon oxynitride, polysilicon, titanium nitride, amorphous carbon, and so forth, and typically formed by a chemical vapor deposition (CVD) method. The under-layer formed by the chemical vapor deposition (CVD) method is excellent in etching selectivity and etching resistance, but there are some drawbacks of a high instrument cost, difficulty in controlling particles, and so on. Therefore, instead of the inorganic under-layer formed by the deposition process, an organic under-layer, which can be formed by a spin coating process, is studied and developed.
A multilayer resist including the organic under-layer typically has a dual-layer structure (dual-layer resist technique) or a triple-layer structure (triple-layer resist technique). In the dual-layer resist technique, an upper-layer is a photoresist layer in which a pattern can be formed, and the lower-layer (resist under-layer) is formed with a hydrocarbon compound which can be etched by using oxygen gas. The resist under-layer should work as a hard mask in etching the underlying substrate, so the resist under-layer should have a high etching resistance. In addition, it is necessary for the resist under-layer to be formed only by a hydrocarbon compound not including silicon atom for the oxygen gas etching process. Also, a wafer on which the resist under-layer is coated can be a wafer having a flat surface, and also can be a wafer on which a circuit pattern is formed. Since the line width and height of the pattern formed on the wafer have the sizes of about several tens to hundreds nanometer, the resist under-layer substance which is coated on the pattern should have a good gap fill property, in order to be effectively and completely filled between patterns having a height difference. Besides the etching resistance and the gap fill property, the resist under-layer is needed to work as an anti-reflective layer for controlling a standing wave in the overlying resist layer and avoiding a pattern collapse when using a KrF or ArF light source. Specifically, it is necessary to control the light reflection from the under-layer to the overlying resist layer to be less than 1%.
In the triple-layer resist technique, an inorganic hard mask intermediate layer (i.e., the second under-layer composed of an inorganic substance) is further provided between the upper-layer (i.e., a photoresist layer) and the resist under-layer (i.e., the first under-layer composed of a hydrocarbon compound). The second under-layer can be a silicon oxide layer, a silicon nitride layer, or a silicon oxynitride (SiON) layer prepared by the chemical vapor deposition (CVD) method at high temperature. Preferably, the second under-layer can be a SiON layer which is also effective as the anti-reflective layer. The thickness of the second under-layer is 5 to 200 nm, preferably 10 to 100 nm. In order to form the second under-layer (particularly, the SiON layer) on the first resist under-layer, the substrate should be heated up to 240 to 500° C. Thus, the resist under-layer (i.e., the first under-layer) should have a thermal stability at the temperature of 240 to 500° C. If the resist under-layer has not a thermal stability at the high temperature (for example, over 400° C.), the resist under-layer may decompose during the formation of the inorganic hard mask intermediate layer (i.e., the second under-layer), and may contaminate the equipments.